Leak detection, materials detection, and qualitative non-destructive testing are well suited to techniques employing fluorescence detection. These techniques rely upon the unique physical property of various materials to fluoresce when excited by certain wavelengths of visible or ultraviolet ("UV") light.
It is a well-known phenomena that electromagnetic energy within the near ultraviolet spectrum of approximately 315 to 400 nanometer wavelengths produces fluorescence in certain materials. That is, the fluorescent materials absorb radiated energy at the near UV or blue wavelengths and re-radiate or emit it at a longer wavelength in the visible spectrum. Thus, when fluorescent material absorbs electromagnetic energy in a specific excitation frequency band in a specific wavelength range, the material can emit electromagnetic energy in a characteristic fluorescent emission frequency band within the visible light spectrum. This phenomena has enabled inspection and detection techniques in which fluorescent dyes, inks or pigments are illuminated by lamps selectively filtered to emit only ultraviolet radiation (invisible to the human eye), and then re-radiate with a high luminescence in the visible spectrum.
For example, the slow leakage of refrigerant from an air conditioning system is difficult to locate by any other means, because the refrigerant escapes as an invisible gas at such low rate and rapid diffusion that the concentration of refrigerant in air near the leak site is difficult to differentiate from that surrounding any other location along the system circulation lines. However, by infusing into the circulating system a small amount of fluorescent dye which is soluble in the refrigerant, the dye is carried out of the system with the refrigerant, and glows brightly at the leak site when the area is swept with a UV lamp.
Currently available inspection lamps employ high intensity light sources operating at very high temperatures to generate a sufficient photon flux for detection applications, and utilize filters to absorb the undesirable wavelengths. These filters are often colored glass which transmit some wavelengths and absorb others. The filters are subjected to significant thermal stress attributed to the high temperature light source and the filter's absorption of light energy. Because of the thermal stress problem, the selection of appropriate materials for a filter are limited. The colored glass filters that are commonly used do not optimize the transmittance of the desirably narrow ultraviolet frequency bandwidth needed to maximize the fluorescence of a particular material.
Another problem with currently available inspection lamps is the tendency for UV colored glass filters to permit transmittance of lower wavelength visible light. These wavelengths interfere with the human eye's perception of light emitted from the fluorescing material. This is a significant limitation in applications where fluorescing material is expected in low concentrations, as in the refrigerant leak example.
Thus, there is a need for an inspection lamp that utilizes a high intensity light source and a filter which will transmit specific wavelengths, while reflecting those which are undesirable. By reflecting undesirable visible light, and transmitting invisible UV light, such a filter can simultaneously maximize a desired transmittance while greatly reducing thermal stress on the filter. A lamp must also be safe to the user and constructed to withstand the rough handling to which it may be exposed. An inspection lamp is also needed that permits the easy substitution of customized filters for differing applications.